The rapid growth of data traffic, and the potential for innovative applications requiring capacities that are orders of magnitude higher than current capacity requirements, motivate a radical new thinking of network structures. An edge-controlled network provides the simplicity and scalability required to construct and control vast versatile networks. Several architectural alternatives can be devised to construct an edge-controlled high-capacity network.
The growth and distribution of data traffic is difficult to quantify and forecast. In addition, the changing nature of traffic and the difficulty of its characterization render network modeling for traffic-engineering purposes impractical. It is, therefore, necessary to rethink the network-design methodology and look for approaches that can lead to networks that reduce or eliminate the need for traffic engineering.
Prior art solutions that aim at circumventing the difficulty of traffic characterization and estimation include agile optical-core networks that scale to several petabits per second (Pb/s). U.S. patent application Ser. No. 09/286,431, filed on Apr. 6th, 1999 and titled SELF-CONFIGURING DISTRIBUTED SWITCH describes a network architecture in which a plurality of high-capacity fast-switching electronic edge nodes are interconnected by an agile wavelength-space-switching optical core. The core node controllers select paths through associated core nodes and reconfigure the paths in response to dynamic changes in traffic loads. The core is reconfigured in response to reconfiguration requests sent from the edge nodes. The reconfiguration requests are based on data traffic volumes, and the steps of reconfiguration include traffic monitoring at ingress edge nodes, communication of traffic-intensity data to core nodes, modifying connections within the core node, and coordinating the channel-switching functions in the edge and core nodes.
In the adaptive channel-switching-core network, each edge node has allocated channels to selected other edge nodes. The number of allocated channels may be modified at reconfiguration time. An edge node accepts new connections based on its current capacity allocation to other edge nodes. The edge-node's controller also monitors its packet queues to other edge nodes and determines whether a change in capacity allocation is warranted. The need for changing capacity allocation is determined at the edge node. The node controller may then request an increment or a decrement in inter-edge-node channel allocation based on occupancy fluctuation.
Each edge node determines its capacity requirements to different sink nodes and communicates them to selected core-node controllers. A selected core-node controller attempts to satisfy the requirements based on free-capacity availability and, possibly, other criteria such as traffic classification. It then returns the scheduling decisions to the edge nodes. At reconfiguration, three functions are implemented: releases (return of resources), capacity-increase requests, and new requests (increase from zero). It is desirable that the traffic load be distributed in a way that equalizes the occupancies of the core nodes.
With adaptive channel switching, traffic streams of low intensity are aggregated in a conventional manner and intermediate switching is performed. A traffic stream with an intensity of less than 0.20 of a channel capacity, for example, can be switched at an intermediate point. The load threshold beyond which a direct-channel is allocated is a design parameter.
Thus, the aforementioned prior-art solutions to network scalability and efficiency confine a connection to a single channel (a single wavelength in a WDM link) and, due to the switching coarse granularity, as an entire channel is switched, a proportion of traffic is transferred from a source node to a sink node through an intermediate edge node. The frequency of reconfiguration is constrained by the propagation delay between the edge nodes and the channel-switching core nodes. A large interval between successive reconfigurations results in coarse granularity. For example, at 10 Gb/s, a reconfiguration interval of 100 milliseconds results in a granularity of 1 gigabit, which is quite high, thus forcing tandem switching for low bit-rate data streams.
In order to further simplify network design and operation, it is desirable to entirely eliminate the need for tandem switching, This simplifies the data scheduling process while maintaining high network efficiency and enables the transfer of data streams of widely-varying capacity requirements.
Fine granularity can be realized by conventional packet switching in the core where some, or all, of the core nodes can be constructed as packet switches thus avoiding the reconfiguration process and the need for time coordination associated with an agile channel-switching core. However, there are several drawbacks in using a packet-switching core:                (1) In order to realize a high-capacity network, a core node must have a large number of ports, 1024 for example, each operating at a high speed, of 10 Gb/s for example. Thus, the packet scheduling process could become unwieldy,        (2) Core switching becomes protocol dependent while, with channel switching, only the source nodes and sink nodes need be protocol aware,        (3) For data streams of high bit rate, for example of several Gb/s, the overhead associated with individual packet routing may be viewed as wasteful considering that the data of the stream is confined to the same physical path anyway and all that is needed is to chain data blocks of the high bit-rate stream with a minimal overhead, and,        (4) extensive buffering may be needed at the core because a core node receives packets from uncoordinated edge nodes.        
Thus, the use of a packet-switching core is not a viable option for a high-capacity high-performance data network.
Circuit-switching has inherent coarse granularity, forcing tandem switching for data streams of low bit rates. With high capacity edge nodes and core nodes, packet switching can eliminate the need for tandem switching, but it has drawbacks as mentioned above. Both adaptive circuit switching and conventional packet switching limit the bit-rate of a data stream below a channel capacity.
In anticipation of applications that would require transferring data streams of very high bit rates, techniques that enable sharing high-capacity links are required. Fiber links, each carrying a large number of wavelengths, are now realizable. Each wavelength can be modulated to carry data at a rate exceeding 10 Gb/s, forming a single channel, and a fiber link may support a multiplicity of channels that can carry data at rates exceeding one terabits per second. This provides a tremendous opportunity for configuring networks that effectively and economically exploit this capability.
In the light of the limitations of the prior art, there is a need for means of pooling channels to increase link utilization and network efficiency, and there is also a need for establishing direct connections, avoiding tandem switching at intermediate edge nodes, in order to simplify network design and control.